One disadvantage of powder coating compositions is that it is difficult to produce small batches of any particular shade rapidly. When the individual powder particles are applied to the substrate surface they remain as discrete entities. With the application of heat these individual particles melt and coalesce, but do not intermix, even when subsequently fused to give a cured film. The effect of this is that if one were to mix red and white powders and apply them, the resultant film would be a red and white speckled film, not a pink one.
The implications of this for the powder manufacturer are many-fold. Firstly, changing from one coloured powder to another, or from one type of powder to another, presents more difficulties than changing liquid paints. The major problem is the avoidance of contamination of one coloured powder by another. This contamination manifests itself as coloured specks in the finished film since, as explained, the colours of finished powders do not blend as wet paints do.
Also, because of the problem of cleaning equipment, small batches of radically different colours are avoided by the manufacturer with large plant which is capable of servicing the large-volume user. Moreover, using conventional powder coatings, it is not possible, for example, to tint a batch.
Improvements in pigment dispersion techniques have made available fluid concentrated colour dispersions which can be added to a range of colourless or white paint bases to prepare liquid paint rapidly in any shade desired, avoiding the need for a paint store to keep several different types of paint each in a wide variety of shades. Such concentrated colour dispersions cannot, however, readily be mixed into powder coatings.
There is therefore a need for a process whereby powder coatings can be rapidly supplied in a wide variety of shades without the need to store all the different shades.
There is also a need for a means of producing a full range of matt finishes applicable to all powder coating compositions. Conventionally, reduction of gloss in powder coatings from full gloss (80-90% measured at a 60.degree. viewing angle) to some other lower level, for example satin gloss (55-65% gloss) or matt (&lt;30% gloss), is achieved by creating a surface which is rough on a microscopic scale. This surface roughness must be sufficient to cause a reduction in the specular reflection from the film by scattering the incident light; if it is visible, however, a texturing effect is achieved in the film. In liquid paints this gloss reduction is usually achieved by the use of pigment and/or filler particles at high volume concentrations. However, this technique cannot be used as the sole route to gloss reduction in powder coatings as a high content of filler particles would lead to too reduced a surface flow during curing.
The procedure used is to set up reactions within the curing film such that two different gelation rates are set up within the curing matrix. With acid-functional polyesters, a fast gelling (reacting) powder and a slow gelling powder may be manufactured separately and mixed after the micronising stage or, more usually, the components are mixed prior to micronising. The faster gelling domains form particles which disrupt the surface flow of the slower gelling portion of the matrix. For production simplicity and economics, a one-component matt is preferred, but the production of an adequate range of matt finishes using a one-component system is not possible in all resin types, and there is no single matting agent that can be used with all resin types to provide a wide variation of gloss levels.
For texture, there are a wide number of agents that can be added to powders to achieve different surface effects, acting by disrupting the flow of the polymer film. For a stippled finish, micronised PTFE (polytetrafluoroethylene) is used. For a wrinkle finish, cellulose acetate butyrate resin (CAB) or acrylate homo- and copolymers may be used; an example of the latter is Acronal 4F (Trade Mark), which is usually added before extrusion as a flow aid, but which in micronised form is used for texturing. For a hammer finish (gross surface depressions), a metallic pigment and texturing agent are used. In addition, texturing agents based on high molecular weight thermoplastics are commonly added to thermoset powder coatings, giving rise to uniformly poor flow across the surface which manifests itself as a texture.
PTFE is preferably added to the premix before extrusion and is fully incorporated into the extrudate in order to minimise product variation through segregation. However, the micronised PTFE has to be prepared in a particular fashion to give batch-to-batch consistency. CAB may be added to the finished powder or, since as a post-additive it is prone to segregation with use, preferably at the premix stage. It does not melt in the extruder, and the inhomogeneous dispersion thereby produced gives rise to the texture. Some texturing agents, however, for example Acronal 4F (T.M.), have to be added post-extrusion since the extrusion process would render them ineffective by mixing them intimately with the continuous phase of the coating system. Powders containing post-additives are susceptible to segregation which causes inconsistency in the texturing effect on application of the powder. There is a need for a powder coating composition giving a textured finish which avoids these problems.
In relation to performance, often a mixture of different film-forming resins is used to provide the desired combination of durability and mechanical properties, or two coatings are applied separately; preferred, however, would be the use of a single coating composition that segregates to provide an enrichment of one polymer having the better durability at the surface of the coating and one having the better mechanical properties near the substrate. Multilayer coatings may be produced using mixed polymer systems by stratification or "surface segregation" of incompatible phases during film formation. Mixtures of, for example, an acrylic polymer and polyester may be used to form a multilayer coating, with the acrylic polymer migrating to the surface of the coating (the air interface) and the polyester forming the lower layer on the substrate. As compared with pure polyesters, the multilayer coating provides improved stain, solvent and UV resistance, for example resistance to degradation by sunlight, and improved durability and weathering performance, and, as compared with pure acrylic, improved mechanical performance.
If powders are made by pre-mixing both the polymers prior to extruding, however, the stratification is not uniform from formulation to formulation or from colour to colour, such that the improvements described cannot be guaranteed. If, alternatively, acrylic and polyester powder coating powders are mixed prior to application, stratification can be complicated by the formation of textured films due to the incompatibility of the two polymer systems. The two powders will also be subject to problems of segregation of the powders in the solid state. There is a need for segregating compositions avoiding these problems.